This application represents a continuous evolution of the subject inventor's inventive technology relating to prestressed tanks or containment vessels. The field of the invention is containment structures and their construction which structures can be used to hold solid,liquids or gases. This invention is particularly useful in the construction of domed structures, utilizing a membrane and circumferential prestressing.
There has been a need for the improved construction of these types of structures as conventional construction has proven difficult and costly. Furthermore, these structures generally do not lend themselves to automation. For example, the current practice has been to construct roofs or domes of such tanks on scaffolding, shoring, framing or decking which is quite costly and time consuming, in contrast to the invention claimed herein where the roof is prefabricated and raised on a cushion of air.
Certain of these conventional structures have utilized prestressed concrete, reinforced concrete or steel tank construction, which are discussed below. Others have utilized Fiber Reinforced Plastic (FRP) and some have utilized inflated membranes.
Turning first to prestressed concrete tanks, their construction have typically utilized prestressing and shotcreting applied by methods set out in detail in U.S. Pat. Nos. 3,572,596; 4,302,978; 3,869,088; 3,504,474; 3,666,189; 3,892,367 and 3,666,190 issued to the subject inventor which are incorporated herein by reference. As set forth in these references, a floor, wall and roof structure is typically constructed out of concrete using conventional construction techniques. The wall is then prestressed circumferentially with wire or strand which is subsequently coated with shotcrete. The machinery used for this purpose is preferably automated, such as that set forth in the above patents. Shotcrete is applied to encase the prestressing and to prevent potential corrosion. As set out in more detail in these patents, and particularly U.S. Pat. No. 5,094,044, which is incorporated herein by reference, prestressing is beneficial in that concrete is not very good in tension but is excellent in compression. Accordingly, prestressing places a certain amount of compression on the concrete so that the tensile forces caused by the fluid inside the tank are countered not by the concrete, but by the compressive forces exerted on the concrete by the prestressing materials. Thus, if design considerations are met, the concrete is not subjected to the substantial tension forces which can cause cracks and subsequent leakage.
Major drawbacks of the above prestressed concrete tank structure are the need for expensive forming of the wall and roof and for substantial wall thickness to support the circumferential prestressing force which places the wall in compression. Furthermore, cracking and imperfections in the concrete structure can cause leakage. Also, conventional concrete tanks are generally not suitable for storage of certain corrosive liquids and petroleum products.
We now turn to tanks constructed using regular reinforcing. This second major category of concrete tanks typically utilize regular reinforcing (in contrast to prestressing), and no membrane. These tanks are inferior to the tanks utilizing circumferential prestressing because, while regular reinforcing makes the concrete walls stronger, it does not prevent the concrete from going into tension, making cracking and leakage an even greater possibility. Typically, reinforcing does not come into play until a load is imposed on the concrete is structure. It is intended to pick up the tension forces because, as previously explained, the concrete cannot withstand very much tension before cracking. Yet reinforcing does not perform this task very well because, unlike circumferential prestressing which preloads the concrete, there are no prestressing forces exerting on the concrete to compensate for the tension asserted by the loading. Moreover, as compared to prestressed concrete tanks, these reinforced concrete tanks require even more costly forming of wall and roof, and even greater wall thicknesses to minimize tensile stresses in the concrete, problems greatly eliminated with the subject invention.
Turning now to inflated membranes, such membranes, have been used for airport structures where the structure consists of the membrane itself. Inflated membranes have also been used to form concrete shells wherein a membrane is inflated and used as a support form. Shotcrete, with or without reinforcing, is sometimes placed over the membrane and the membrane is removed after the concrete is hardened. Another form of this construction is exemplified by conventional "Binishell" structures. Information regarding such structures is in the Disclosure Statement and in U.S. Pat. No. 3,462,521. These structures are constructed by placing metal springs, and regular reinforcing bars over an uninflated lower membrane. Concrete is then placed over the membrane and an upper membrane is placed over the concrete to prevent it from sliding to the bottom as the inflation progresses. The inner membrane is then inflated while the concrete is still soft. After the concrete has hardened, the membranes are typically removed. A major drawback of the afore-described conventional structures is the high cost connected with reinforcing and waterproofing them for liquid storage. Moreover, with regard to the "Binishell" structures, because of the almost unavoidable sliding of the concrete, it is difficult if not impossible to avoid honeycombing of the concrete and subsequent is leaks. Also Bini does not teach the utilization of membranes in conjunction with circumferential prestressing, in contrast to using mere reinforcing. As a result, these structures have not been very well received in the marketplace and have not, thus far, displaced the more popular and commercially successful steel, reinforced concrete and prestressed concrete tanks and containment vessels. Substantial improvements to these types of membrane structures are set out in U.S. Pat. Nos. 4,879,959; 5,134,830; 4,776,145; 5,094,044 issued to the subject inventor which are incorporated by reference, but which do not accomplish the advantages of the subject invention.
Another general category of existing tanks are those made of fiberglass. These fiberglass tanks have generally been small in diameter, for example, in contrast to the prestressed or steel tanks that can contain as many as 30 million gallons of fluid. The cylindrical walls are often filament-wound with glass rovings. To avoid strain corrosion, (a not very well understood condition wherein the resins and/or laminates fracture, disintegrate or otherwise weaken) the tension in fiberglass laminates is typically limited to 0.001 in/in (or 0.1%) strain by applicable building codes or standards and by recommended prudent construction techniques. For example, the American Water Works Association (AWWA) Standard for Thermosetting Fiberglass, Reinforced Plastic Tanks, Section 3.2.1.2 requires that "the allowable hoop strain of the tank wall shall not exceed 0.0010 in/in." A copy of this standard is provided in the concurrently filed Disclosure Statement. Adhering to this standard means, for example, that if the modulus of elasticity of the laminate is 1,000,000 psi, then the maximum design stress in tension should not exceed 1,000 psi (0.001.times.1,000,000). Consequently, large diameter fiberglass tanks have required substantially thicker walls than steel tanks. Considering that the cost of fiberglass tanks has been close to those of stainless steel, another common type of tank, and considering the above strain limitation, there are not believed to have been any viable large diameter fiberglass tanks built world-wide since fiberglass became available and entered the market some 35 years ago. Another reason why large fiberglass tanks have not been viable, is the difficulty of operating and constructing the tanks under field conditions, water tanks, for example are often built in deserts, mountaintops and away from the pristine and controlled conditions of the laboratory. Resins are commonly delivered with promoters and catalysts for a certain fixed temperature, normally room temperature. However, in the field, temperatures will vary substantially. Certainly, variations from 32.degree. F. to 120.degree. F. may be expected. These conditions mean that the percent of additives for promoting the resin and the percent of catalyst for the chemical reaction, which will vary widely under those temperature variations, need to be adjusted constantly for the existing air temperatures. Considering that these percentages are small compared to the volume of resin, accurate metering and mixing is required which presents a major hurdle to on-site construction of fiberglass tanks, The above problems have been remedied to a great extent by the teachings of the undersigned inventor's U.S. Pat. Nos. 4,879,856; 5,134,830; 4,884,747; 5,076,495 and 5,092,522 which are hereby incorporated by reference, and regarding which the subject patent represents a further evolution and improvement. There have also been problems with seismic anchoring of the above tanks, some of which have been solved by the techniques and apparatus disclosed in Mr. Dykman's U.S. Pat. Nos. 5,105,590 and 5,177,919, which are also hereby incorporated by reference.